3
Chapter 2
Theory of Operation
General
This load resistor utilizes an external water supply for the direct cooling of the
resistor element. By using this technique, the need for an intermediate dielec-
tric fluid to transfer the heat generated in the resistor element has been elim-
inated, reducing the physical size of the load to a virtual minimum. This
simplified system allows the use of the loads in more varied environments,
and attachment in any orientation.
Heat Transfer
The 50 ohm resistor consists of a high temperature substrate tube with a
deposited resistive film. The heat generated by absorption of RF power is
transferred from the heated film to the water flowing over it, through a
restricted chamber surrounding the resistor body. This water, first diverted to
the front of the load resistor by a special inside tube, passes over the entire
length of the resistor and is discharged through the sealed water chamber at
the rear. The dielectric characteristics and distinctive design of these enclo-
sures provide a very accurate 50 ohm termination over the specified frequency
range of this load (1000 Hz to 900 MHz).
The absence of intermediate cooling fluids considerably simplifies the con-
struction and sealing of this unit. It can be readily disassembled in the field
for resistor element replacement (see “Maintenance” on page 9).
Because there is practically no heat transfer to the outer housing of the load,
the housing remains at a cool ambient temperature even under full power con-
ditions. Virtually all of the power input to the load is transformed into heat
which is carried away by the cooling water. Therefore, the differential in out-
put and input temperatures of the water, times the amount of flow, consti-
tutes an accurate gage of the power consumed by the load. The amount of this
power dissipation may be calculated from the following formula:
where:
In °F the formula is:
T
2
InletWaterTemperatureInCelcius
=
GPM
WaterFlowInGallonsPerMinute
=
T
1
OutletWaterTemperatureInCelcius
=
P
PowerInKilowatts
=
P
0.263
T
1
T
2
–
(
)
GPM
=
P
0.146 0.263
(
)
T
1
T
2
–
(
)
GPM
=
P
PowerInKilowatts
=
T
1
OutletWaterTemperatureInFahrenheit
=
T
2
InletWaterTemperatureInFahrenheit
=
GPM
WaterFlowInGallonsPerMinute
=
where:
Summary of Contents for TERMALINE 8745 Series
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